This invention relates to integrated power generation and air purification and conditioning systems for stationaryand mobile applications. Specifically, an air purification subsystem may be installed in a fuel cell system across a cathode gas diffusion layer or along a cathode flow path to enhance air purification by utilization of fuel cell operating conditions.
A fuel cell is a device which converts chemical energy of a fuel into electrical energy, typically by oxidizing the fuel. In general a fuel cell includes an anode and a cathode separated by an electrolyte. When fuel is supplied to the anode and oxidant is supplied to the cathode, the cell electrochemically generates a useable electric current which is passed through an external load. The fuel typically supplied is hydrogen and the oxidant typically supplied is oxygen. In such cells, oxygen and hydrogen are combined to form water and to release electrons. The chemical reaction for a fuel cell using hydrogen as the fuel and oxygen as the oxidant is shown in equation (1).
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(1)
This process occurs through two half-reactions which occur at the electrodes: Anode Reaction
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(2)
Cathode Reaction
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(3)
In the anode half-reaction, hydrogen is consumed at the fuel cell anode releasing protons and electrons as shown in equation (2). The protons are injected into the fuel cell electrolyte and migrate to the cathode. The electrons travel from the fuel cell anode to cathode through an external electrical load. In the cathode half-reaction, oxygen, electrons from the load, and protons from the electrolyte combine to form water as shown in equation (3).
The directional flow of protons, such as from anode to cathode, serves as a basis for labeling an xe2x80x9canodexe2x80x9d side and a xe2x80x9ccathodexe2x80x9d side of the fuel cell.
Fuel cells are classified into several types according to the electrolyte used to accommodate ion transfer during operation. Examples of electrolytes include aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate, stabilized zirconium oxide, and solid polymers, e.g., a solid polymer ion exchange membrane.
An example of a solid polymer ion exchange membrane is a Proton Exchange Membrane (hereinafter xe2x80x9cPEMxe2x80x9d) which is used in fuel cells to convert the chemical energy of hydrogen and oxygen directly into electrical energy. A PEM is a solid polymer electrolyte which when used in a PEM-type fuel cell permits the passage of protons (i.e.,H+ ions) from the anode side of a fuel cell to the cathode side of the fuel cell while preventing passage of reactant fluids such as hydrogen and oxygen gases.
A PEM-type cell includes an electrode assembly disposed between an anode fluid flow plate and a cathode fluid flow plate. An electrode assembly usually includes five components: two gas diffusion layers; two catalysts; and an electrolyte. The electrolyte is located in the middle of the five-component electrode assembly. On one side of the electrolyte (the anode side) a gas diffusion layer (the anode gas diffusion layer) is disposed adjacent the anode layer, and a catalyst (the anode catalyst) is disposed between the anode gas diffusion layer and the electrolyte. On the other side of the electrolyte (the cathode side), a gas diffusion layer (the cathode gas diffusion layer) is disposed adjacent the cathode layer, and a catalyst (the cathode catalyst) is disposed between the cathode gas diffusion layer and the electrolyte.
Several PEM-type fuel cells may be arranged as a multi-cell assembly or xe2x80x9cstack.xe2x80x9d In a multi-cell stack, multiple single PEM-type cells are connected together in series. The number and arrangement of single cells within a multi-cell assembly are adjusted to increase the overall power output of the fuel cell. Typically, the cells are connected in series with one side of a fluid flow plate acting as the anode for one cell and the other side of the fluid flow plate acting as the cathode for an adjacent cell.
The anode and cathode fluid flow plates are typically made of an electrically conductive material, typically metal or compressed carbon, in various sizes and shapes. Fluid flow plates may act as current collectors, provide electrode support, provide paths for access of the fuels and oxidants to the electrolyte, and provide a path for removal of waste products formed during operation of the cell.
The cell also includes a catalyst, such as platinum on each side of the electrolyte for promoting the chemical reaction(s) that take place in the electrolyte in the fuel cells. The fluid flow plates typically include a fluid flow field of open-faced channels for distributing fluids over the surface of the electrolyte within the cell.
Fluid flow plates may be manufactured using any one of a variety of different processes. For example, one technique for plate construction, referred to as xe2x80x9cmonolithicxe2x80x9d style, includes compressing carbon powder into a coherent mass which is subjected to high temperature processes to bind the carbon particles together, and to convert a portion of the mass into graphite for improved electrical conductivity. The mass is then cut into slices, which are formed into the fluid flow plates. Typically, each fluid flow plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions.
Fluid flow plates may also have holes therethrough which when aligned in a stack form fluid manifolds through which fluids are supplied to and evacuated from the stack. Some of the fluid manifolds distribute fuel (such as hydrogen) and oxidant (such as air or oxygen) to, and remove unused fuel and oxidant as well as product water from, the fluid flow fields of the fluid flow plates. Additionally, other fluid manifolds circulate coolant to control the temperature of the stack. For example, a PEM fuel cell stack may be maintained in a temperature range of from 60xc2x0 C. to 200xc2x0 C. The temperature of the anode and cathode exhaust streams may also be within this range as they leave the fuel cell. Cooling mechanisms such as cooling plates are commonly installed within the stack between adjacent single cells to remove heat generated during fuel cell operation.
PEM fuel cell systems using hydrogen as a fuel may include a fuel processing system such as a reformer to produce hydrogen by reacting a hydrocarbon such as natural gas or methanol. Many such fuel processing systems are well known in the art. Where a reformer is used, the reformed fuel gas is referred to as reformate, and may typically contain predominantly hydrogen, carbon dioxide and water. In some cases, reformate may also a relatively small amount of carbon monoxide. Since carbon monoxide, even in trace amounts, acts as a poison to most fuel cell catalysts, for example platinum-based catalysts, methods have been developed to minimize or eliminate carbon monoxide in reformate streams. Such methods include, for example, using a preferential oxidizer system to convert carbon monoxide into non-poisoning carbon dioxide, or optimizing fuel processor operating conditions such as temperature and air flow to minimize the production of carbon monoxide in the reformer.
Typically only a portion of the reactants (e.g., reformate containing hydrogen on the anode side, and air containing oxygen on the cathode side) flowing through a fuel cell will react. For example, the amount of reactants in the anode and cathode streams that are reacted may depend on factors including temperature, pressure, residence time, and catalyst surface area. For this reason, it may be desirable to feed excess reactants to a fuel cell in order to increase the reaction level to a point corresponding to a desired power output of the fuel cell. For example, it may be that 100 standard liters per minute (slm) of hydrogen must be reacted in a fuel cell to achieve a desired power output, but it is determined that 140 slm of hydrogen must be fed to the fuel cell to achieve this reaction of 100 slm of hydrogen. This system may be said to be running at 40% excess hydrogen at the anode inlet. In other terminology, this system may also be characterized as running at a stoichiometry of 1.4. For similar reasons, it may be desirable to supply the cathode side of the fuel cell with an excess of oxidant. The stoichiometry of the anode and cathode flows may be selected independently.
It will thus be appreciated that by reacting hydrogen from the anode stream, the fuel cell provides an anode exhaust stream that is concentrated in its non-hydrogen components. Likewise, by reacting oxygen from the cathode stream, the fuel cell provides a cathode exhaust stream that is concentrated in its non-oxygen components.
In most environments, fuel cell cathode air streams will not contain significant carbon monoxide levels because carbon monoxide is not generally present in fresh atmospheric air. However, in polluted environments, such as might be seen by fuel cells, for example, in or near automotive and commercial environments, the ambient air fed to the fuel cell may contain carbon monoxide as well as other air contaminants such as ozone. Carbon monoxide is a well known poison to humans and animals. Ozone is known to cause a variety of health problems including lung damage. Tropospheric ozone is also known to cause substantial damage to agricultural crops. Various other organic and non-organic pollutants may also be present in such environments, such as those emitted from automotive exhaust and the evaporation of organic solvents.
In general, in one aspect, the invention provides a fuel cell system with an air purification subsystem located along the cathode flow path, wherein the air purification subsystem utilizes heat from the fuel cell to react air pollutants. In this context, the cathode flow path refers to the oxidant flow through the fuel cell system, starting with the oxidant inlet (e.g., an air blower or compressor), and ending with the cathode exhaust as it leaves the fuel cell system. In one possible embodiment, the air purification subsystem may include a multi-purpose catalys bed suitable for converting carbon monoxide into carbon dioxide, and converting ozone into diatomic oxygen. The air purification subsystem may also be effective to oxidize organic pollutants. In other embodiments, the catalyst bed may be selected to abate a specific air pollutant. As examples, suitable catalysts may include precious metals and alloys thereof, being supported by conventional means known in the art such as alumina and zeolite monoliths, and carbon black catalyst support systems. In one possible embodiment, the electrodes of the fuel cell may include a multi-purpose platinum-based catalyst structure and composition that is optimized to provide enhanced air purification in addition to the hydrogen oxidation required to power the fuel cell. In general, the air purification subsystem may be located up or downstream of the fuel cell, or along the cathode flow plates of the fuel cell, or on the fuel cell cathode or cathode side gas diffusion layer.
In another aspect, the invention contemplates systems for providing purified air to an interior air space of a building or a vehicle, as examples, by treating the cathode air flow of a fuel cell system with the catalytic air purification subsystem, and then supplying the interior air space with the cathode exhaust from the fuel cell. The decrease in oxygen in the cathode stream may be referred to as the cathode oxygen depletion. Additional embodiments may include a means for selecting the cathode flow rate of the fuel cell system to control the cathode oxygen depletion of the cathode exhaust. For example, the stoichiometry of the cathode flow may be set to provide a cathode oxygen depletion of less than 10% or less than 5%, as examples. A by-pass may also be provided to control the flow of the cathode exhaust into the interior air space. A heat exchanger may also be provided to control the temperature of the cathode exhaust. A dehumidifier may also be provided to control the humidity of the cathode exhaust.
In another aspect, the air purification subsystem can include a catalyst bed adapted to react carbon monoxide at an operating temperature of the fuel cell system. For example, carbon monoxide may be reacted at a temperature of less than 200xc2x0 C. for certain fuel cell systems, or less than 100xc2x0 C. or less than 80xc2x0 C. for other fuel cell systems. In another aspect, the air purification subsystem can include a catalyst bed adapted to react ozone at such operating temperatures. As an example, for a system adapted to focus on carbon monoxide removal, it may be desirable for the purification subsystem to be located in the cathode flow path at a point upstream from the fuel cell. As another example, for a system adapted to focus on ozone removal, it may be desirable for the purification subsystem to be located in the cathode flow path at a point downstream from the fuel cell.